So-called air-based air conditioning systems are usually used at present in commercial aircraft to air-condition the aircraft cabin. An aircraft air conditioning system serves to cool the aircraft cabin, which would otherwise be overheated due to thermal loads, such as, e.g. insulation, body heat of the passengers and waste heat from equipment present on board the aircraft. The aircraft air conditioning system in addition adequately supplies fresh air into the aircraft cabin to ensure that a prescribed minimum proportion of oxygen is present in is the aircraft cabin. The structure and the operating principle of an air-based air conditioning system which is commonly used today are illustrated in FIG. 1.
In the aircraft air conditioning system which is shown in FIG. 1 hot process air, which is either bled off the aircraft engines or produced by compressors formed separately from the engines when the aircraft is in flight, is supplied to an air conditioning unit. The process air, which is supplied at a high temperature and under high pressure, is treated in the air conditioning unit upon flowing through a heat exchanger unit as well as a compression and expansion unit such that it leaves the compression and expansion unit as expanded and cooled process air. The expanded and cooled process air is supplied as fresh air to a mixing chamber, where it is mixed with recirculation air removed from the aircraft cabin. The air mixture of fresh air and recirculation air which is produced in the mixing chamber is finally routed into the aircraft cabin, and the mixed air mass flow which is supplied to the aircraft cabin from the mixing chamber should be as constant as possible in order to obtain optimal cabin comfort. Consequently the recirculation air proportion should be increased accordingly when the fresh air proportion of the mixed air is reduced and vice versa. However, when setting the fresh air proportion of the mixed air it is necessary to observe very strict regulations of the aeronautical authorities, which set down in detail the fresh air mass flow to be supplied to an aircraft cabin for different aircraft operating states, for example in accordance with the number of passengers on board the aircraft.
In the heat exchanger unit of the air conditioning unit, which may comprise a plurality of heat exchangers, the hot process air which is supplied to the heat exchanger unit is cooled through the transfer of thermal energy to cold aircraft ambient air flowing through the heat exchanger unit. The aircraft ambient air is supplied to the heat exchanger unit through a ram air inlet duct at a low temperature and under low pressure. After flowing through the heat exchanger unit, the ambient air, which is heated by delivering its cooling energy to the hot process air, leaves the aircraft through a ram air outlet duct.
The greater the quantity of hot process air which must be provided as engine bleed air by the engines of an aircraft, the higher the fuel consumption of the aircraft. An increase in the fuel consumption when there is an increase in the process air mass flow also occurs when using compressors which are formed separately from the engines, as the electrical energy which is required to drive the compressors must be provided by the engines. This correlation is illustrated in the diagram which is represented in FIG. 2. Therefore, in order to minimise the fuel consumption when an aircraft is in flight, the process air mass flow which is supplied to the air conditioning unit should be kept as small as possible.
Given a constant temperature of the recirculation air which is supplied into the mixing chamber of the aircraft air conditioning system from the aircraft cabin, the temperature of the mixed air which is directed into the aircraft cabin from the mixing chamber is determined by the fresh air mass flow which is routed into the mixing chamber as well as the temperature of the fresh air which is fed to the mixing chamber. Therefore, given a reduction in the process air mass flow which is supplied to the air conditioning unit for treatment and a reduction resulting directly therefrom of the fresh air mass flow which is available for supplying into the mixing chamber, with a constant cooling capacity requirement of the aircraft cabin, the temperature of the fresh air which is routed into the mixing chamber must be reduced accordingly if the air mixture which is produced in the mixing chamber of the aircraft air conditioning system is to leave the mixing chamber at a constant low temperature. Decreasing the fresh air temperature to a lower level requires an increase in the cooling capacity of the air conditioning unit.
The cooling capacity of the air conditioning unit is determined by the cooling capacity provided by the compression and expansion unit as well as the cooling capacity provided by the heat exchanger unit. However the maximum cooling capacity which is to be produced by the compression and expansion unit is as a rule limited by the system pressure, i.e. the pressure at which the process air is supplied to the compression and expansion unit by the engines or the compressors formed separately from the engines. An increase in the cooling capacity of the air conditioning unit is therefore in most cases only possible by increasing the cooling capacity which is provided by the heat exchanger unit.
An increase in the cooling capacity which is provided by the heat exchanger unit of the air conditioning unit can be effected by increasing the mass flow of the ambient air which is routed as cooling air through the heat exchanger unit. For this it is necessary to open up larger flow cross sections of the ram air inlet duct and/or the ram air outlet duct, which is usually effected by opening corresponding flaps which control the flow cross sections of the ram air inlet duct and/or the ram air outlet opening. However, on account of the fact that the flaps project out of the aircraft outer skin in the open state, the opening of the flaps controlling the flow cross sections of the ram air inlet duct and/or the ram air outlet duct increases the aerodynamic drag and therefore the fuel consumption of the aircraft. This correlation is illustrated in the diagram which is represented in FIG. 3.
Given a constant cooling capacity requirement of the aircraft cabin, the ambient air mass flow through the heat exchanger unit of the air conditioning unit must be increased all the more as the process air mass flow is reduced in order to compensate for a reduced process air mass flow. In other words, the flaps controlling the flow cross sections of the ram air inlet duct and/or the ram air outlet duct must be brought into an even further open position the less process air is provided for the air conditioning unit by the engines of the aircraft or the compressors formed separately from the engines. On the other hand, the flaps can be closed all the more further as the process air mass flow provided for the air conditioning unit by the engines of the aircraft or the compressors formed separately from the engines increases. This correlation is illustrated in the diagram which is represented in FIG. 4.
To summarise, it can therefore be said that a reduction in the process air mass flow which is bled off the engines of the aircraft or provided by compressors formed separately from the engines does in fact directly enable the fuel consumption of the aircraft to be decreased. However the increase in the ambient air mass flow through the heat exchanger unit of the air conditioning unit which is required to compensate for the reduced process air mass flow when the cooling capacity requirement of the aircraft cabin is constant results in an increase in the fuel consumption of the aircraft.
The object of the invention is to provide a method and a system for controlling an aircraft air conditioning system which enable the fuel consumption of the aircraft to be optimized in different operating situations of the aircraft air conditioning system.